1. Field of the Invention
The present invention relates generally to low-detectability transmission between a transmitter and a receiver, and more particularly, to a low-detectability sensor communication.
2. Prior Art
There are many situations where it is desired to insert a surveillance sensor in a location which is inaccessible to the user after deployment, and in which it is necessary to conceal the presence of the sensor from those under surveillance, the most obvious example being sensors located in enemy territory. Since the sensor is inaccessible, the sensor must transmit some form of wireless electronic communication for the data to be recovered. This however, requires that the sensor be capable of emitting electromagnetic energy, with the result that the sensor may be detected by those under surveillance, and destroyed.
There is therefore a conflict between the requirement for data recovery, and the requirement that the sensor be “stealthy”. It is important to note that the stealth requirement is quite different from the security of the data transmitted by the sensor. The issue here is not the sensor data security (for which encryption is the appropriate tool), but about the detectability of the sensor itself. However, as will be discussed below, encryption can be important in the methods of the present invention, if not for different reasons.
The fundamental problem is that the emissions from the sensor will, if directional antennas and triangulation are used by opponents, eventually enable the sensor to be located and destroyed. Several possible solutions have been proposed for this problem, all of which have drawbacks. These can be summarized as two approaches to “hiding” the sensor emissions in three domains: the three domains are time, space, and frequency (including modulation key), and the two approaches are either to concentrate the energy in the domain, possibly with pseudo-random changes in the location, or to spread it over the domain so that its intensity at any point is low enough to escape detection.
As an example of this, the normal spatial mode of communication is omnidirectional; but if the sensor concentrates its radiation vertically upwards it is unlikely to be picked up by opponents. The disadvantage is that a receiver must remain on station to receive the data. This can make it relatively easy to locate and destroy, which has the same end effect as the destruction of the sensor. In this case, pseudo-random changes in the direction are probably not practical in terms of receiver location and costs.
The choice between diversity and concentration in the other two domains (time and frequency/modulation) is much more complex, with numerous approaches having been proposed; for example, sensing the local noise environment, and transmitting in regions of the frequency spectrum where the ambient noise is low, at an intensity which brings the total spectral density in the low-noise regions just up to the average noise level, in effect “whitening” the noise as seen by the opponent. A disadvantage of this, which is addressed by the proposed method, is that the noise spectrum seen by the receiver is likely not to be the same as that seen by the transmitter, and so the receiver would not know what frequency band to demodulate.